With only 20% left there's only so much that miners can mine with present day mining hardwares and it'll get slower and slower to mint new Bitcoins as the block rewards get smaller. This will eventually make Bitcoins rare to procure, making new coins less avialable to traders / buyers. Now this could eventually drive the price up sharply cause this would make Bitcoin like gold where there's limited supply of gold and more demand. Miner could hence profit just as much as they are now. This trend will continue till all Bitcoins have been mined and then they can only earn from fees for mining the blocks.